Which of these numbers is prime? ${9,\ 15,\ 27,\ 72,\ 73}$
A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 15 are 1, 3, 5, and 15. The factors of 27 are 1, 3, 9, and 27. The factors of 72 are 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, and 72. The factors of 73 are 1 and 73. Thus, 73 is a prime number.